RFID Antenna

ABSTRACT

An antenna particularly suitable for use in radio frequency identification (RFID) transponders. First and second half portions each include nominally straight conductive sections primarily defining a radiating characteristic and nominally spiral conductive sections creating a positive reactive characteristic of the antenna. The straight conductive sections have feed points for connecting the antenna into the RFID transponder, and further connect with the respective spiral conductive sections. The first and said second half portions characterize the antenna as being a dipole type, and adding an optional loop section connecting the straight conductive sections can further characterize it as being of a folded dipole type.

TECHNICAL FIELD

The present invention relates generally to electrical communications,more particularly to condition-responsive indicating systems, yet moreparticularly to detectable devices or tags, and still more particularlyto where a detectable device or tag has a particular aerial wavecollector structure for sending or receiving signals.

BACKGROUND ART

Radio Frequency Identification (RFID) systems are in wide and growinguse today. FIG. 1 (prior art) is a schematic diagram depicting anexample RFID system including an interrogator and a transponder. In someRFID systems the “identifying” information is written into thetransponder during manufacture and never changed, making theinterrogator merely a reader. In other systems, however, the identifyinginformation in the transponder can be changed and the interrogator usedcan then be both a reader and a writer. In view of this, the term“interrogator” is used herein to generically mean a reader, a writer, orboth. The terms “transponder” and “tag” have become almost synonymousand are used herein as such.

RFID tags are generally classified by whether they are active orpassive. A passive-type RFID tag includes transponder circuitry and anantenna, while an active-type RFID tag additionally includes a powersource, such as a battery, fuel-cell, or some equivalent. The circuitryin transponders today is usually embodied in a single integratedcircuit, hence the term “transponder chip” is often used.

FIG. 1 depicts a passive-type RFID tag. Such tags are used primarily asthe examples in this discussion because they are the most commonly usedtype today and because they will usually benefit more noticeably by usewith the present invention. Nonetheless, it should be appreciated thatmany active-type RFID designs will also benefit by use of the invention.

A passive-type RFID tag extracts energy from an externally providedradio frequency (RF) wave. Typically this RF wave is an interrogationsignal being used to excite the antenna to read or write information inthe tag. The lack of a built-in energy source tends to make passive RFIDtags cheaper to manufacture, longer lasting, and more reliable. Thisalso tends to make them environmentally friendly, because they do notinclude the environmentally unfriendly substances typically used inpower sources. The lack of a built-in energy source, however, alsolimits the effective operating range of a passive RFID tag with respectto the given energy in an interrogation signal. For example, to increaseoperating range or if signal propagation between an interrogator and atag is somehow limited, it follows that the interrogator being used witha passive RFID tag will have to radiate the interrogation signal at ahigher power level to accomplish the task at hand.

Unfortunately, simply increasing the power level of an interrogationsignal to insure successful interrogator-tag communications is notalways possible. For instance, simple inefficiency can exacerbateproblems such as battery life and heat dissipation in the interrogator.Of more serious concern, RF energy radiation intended for one system caninterfere with other electronic systems and, in extreme cases, can beunsafe for biological systems, e.g., humans, animals, plants, etc. Forthis reason, most governments limit RF energy radiation levels, and theUnited States and both Europe are notable in this respect.

With reference again to FIG. 1, the exemplary passive-type RFID tagdepicted there includes an antenna, a matching network, a modulator, arectifier, and a logic sub-circuit. The antenna is a dipole-type, as isfrequently used in RFID systems today. The matching network is shown ina dashed outline because it is optional, as discussed below. Thecircuitry depicted in the matching network, modulator, and rectifier inFIG. 1 is merely representative, and no circuitry is depicted in thelogic sub-circuit because such can vary considerably and is notparticularly germane to this discussion. The antenna and the matchingnetwork usually must be implemented in discrete components, but themodulator, rectifier, and logic sub-circuit are usually implementedtoday as a single integrated circuit “transponder chip”.

Temporarily ignoring the matching network, the antenna is connected tothe “front end” of the transponder chip and the electromagnetic field ofan interrogation signal that impinges on the antenna must produce anoutput signal having a voltage above a given threshold before thetransponder chip can rectify it.

Rectifying the received interrogation signal can serve multiplepurposes. In a passive RFID tag it provides the power needed to operatethe logic sub-circuit, and ultimately also the modulator that permitsthe transponder to “send” its identifying information back to aninterrogator as backscatter radiation. Additionally, rectificationdemodulates the interrogation signal if it is providing information tothe transponder. This is so if the logic sub-circuit is beingprogrammed, either with the identifying information that the transponderwill “reply” with when later read, or with any other programming thatthe logic sub-circuit can accept. Additionally, the RF carrier of aninterrogation signal may include some indication to transponders that itworks with that it is a valid interrogation signal, e.g., a particularsub-carrier frequency. This permits the transponder to remain silentwhen energized by other, invalid RF signals.

To maximize the voltage produced by the antenna, and to thus increasethe energy provided to the transponder, the impedance between theantenna and the transponder should match at the operating frequency ofthe particular interrogation signal being used. One known approach toimproving this impedance matching is to utilize circuits of eitherdiscrete components, e.g., inductor and capacitor networks ordistributed elements such as microstrip structures. The inductor andcapacitor matching network shown in FIG. 1 is an example. Unfortunately,these approaches are often undesirable because they increase the cost,complexity, and size of the RFID tag and decrease its efficiency.

It would clearly be very desirable be able to produce an RFID tag withan antenna that is directly matched to the RF circuitry of thetransponder chip, to make the most of interrogation signals that arereceived and to increase the possible operating range between the RFIDtag and the interrogator it is used with. Unfortunately, this has untilnow not been possible without accepting other undesirable consequences.

To achieve optimal impedance matching the transponder antenna should bethe conjugate of the impedance of the transponder chip. However, due toits use of diodes and capacitors, the rectifier at the front end of thetransponder chip has an input impedance that is a complex value innearly all cases, comprising both a positive real component and anegative imaginary component. The impedance of the antenna thereforeshould have the same real component and a positive imaginary component.This normally requires an increase in antenna size, in comparison with aresonant dipole, and that especially conflicts with the usual designgoals of reducing overall transponder size and cost.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide animproved antenna structure and methods for using such in radio frequencyidentification (RFID) transponders (or “tags”).

Briefly, one preferred embodiment of the present invention is an antennafor use in a radio frequency identification (RFID) transponder. A firstand a second half portions are provided that each include a straightconductive section and a spiral conductive section.

The straight conductive sections primarily define a radiatingcharacteristic of the antenna and the spiral conductive sections createa positive reactive characteristic of the antenna. The straightconductive sections each have a feed point where the antenna can beconnected into the RFID transponder. The straight conductive sectionseffectively connect with their respective spiral conductive section,thereby having the first and second half portions characterize theantenna as being of a dipole type.

Briefly, another preferred embodiment of the present invention is alsoan antenna for use in a radio frequency identification (RFID)transponder, having all of the elements of the just recited embodimentand additionally having a loop section connecting the straightconductive sections of the first and second half portions and thusfurther characterizing the antenna as being of a folded dipole type.

An advantage of the present invention is that it is particularlysuitable for use in passive-type RFID tags by virtue of its highefficiency. Although, the present invention is also quite suitable andbeneficial for use in many active-type RFID tag designs.

Another advantage of the invention is that it can be implemented as asimple structure having a small size, thus making it suitable to servein applications where a size is a limitation and making its manufactureand materials requirements quite economical.

Another advantage of the invention is that it permits a more omnidirectional antenna pattern than many existing designs, facilitatingconsistent and reliable transponder detection.

And another advantage of the invention is that it matches well with therectifier of the RFID device it is used in without employing additional,discrete circuit elements.

These and other objects and advantages of the present invention willbecome clear to those skilled in the art in view of the description ofthe best presently known mode of carrying out the invention and theindustrial applicability of the preferred embodiment as described hereinand as illustrated in the figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The purposes and advantages of the present invention will be apparentfrom the following detailed description in conjunction with the appendedfigures of drawings in which:

FIG. 1 (prior art) is a schematic diagram depicting an example RFIDsystem including an interrogator, transponder, and representativecircuitry therein.

FIG. 2 depicts a dipole type antenna in accord with the presentinvention.

FIG. 3 depicts another dipole type antenna in accord with the presentinvention.

FIG. 4 depicts a folded-dipole type antenna in accord with the presentinvention.

And FIG. 5 depicts another folded-dipole type antenna in accord with thepresent invention.

In the various figures of the drawings, like references are used todenote like or similar elements or steps.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention is an antenna for use inRadio Frequency Identification (RFID) transponders. As illustrated inthe various drawings herein, and particularly in the views of FIG. 2-5,preferred embodiments of the invention are depicted by the generalreference characters 10, 30, 50, and 70.

FIG. 2 depicts an antenna 10 in accord with the present invention thatconsists of two sections 12, 14. The first section 12 consists of themainly radiating components, here straight conductive traces 16, whilethe second section 14 consists of substantially reactive components,here spiral conductive traces 18. The spirals shown in the figuresherein are typical, and might have an arbitrary number of turns,lengths, widths, etc.

In the characteristic manner of dipole-type antennas, the antenna 10here has a left dipole half 20 mirrored by a right dipole half 22. Thesehalfs of the antenna 10 do not correspond with and should not beconfused with the sections 12, 14. The left dipole half 20 of theantenna 10 includes both a first section 12 and a second section 14, asdoes the right dipole half 22. Each first section 12 includes a feedpoint 24, for connecting the antenna 10 to a transponder chip (notshown). As shown, each first section 12 also, directly or indirectly,connects to its respective second section 14.

In use, the spiral structures of the second sections 14 (the spiralconductive traces 18 in FIG. 2) substantially increase the imaginarypart of the impedance of the antenna 10. The use of such a spiralstructure has the particular advantage over traditional meander-linestructures as it creates the positive reactive component of theimpedance in a more compact size.

FIG. 3 depicts another antenna 30, extending the dipole type principlesof the antenna 10 in FIG. 2. The antenna 30 also consists of twosections 12, 14, but the second section 14 here has two parts, asub-section 14 a and a sub-section 14 b, that include four spiralconductive traces 18.

FIG. 4 depicts an antenna 50 in accord with the present invention thatalso consists of two sections 52, 54. The first section 52 consists ofthe mainly radiating components (straight conductive traces 56) and thesecond section 54 consists of the substantially reactive components(spiral conductive traces 58). The antenna 50 here also has a leftfolded dipole half 60 and a mirrored light folded dipole half 62, witheach first section 52 including a feed point 64 for connection to atransponder chip and with each first section 52 also, directly orindirectly, connecting to its respective second section 54. Unlike thedipole antenna 10 of FIG. 2, however, the antenna 50 of FIG. 3 has aloop section 66 connecting the dipole halfs 60, 62 in the manner thatconductive loops are traditionally used to create folded dipole-typeantennas.

In use, here as well, the spiral structures of the second sections 54(the spiral conductive traces 58 in FIG. 3) substantially increase theimaginary part of the impedance of the antenna 50 and distinguish itover traditional structures, such as meander-line based designs.

FIG. 5 depicts another antenna 70, extending the folded-dipole typeprinciples of the antenna 50 in FIG. 4. The antenna 70 also consists oftwo sections 52, 54, but the second section 54 here has two parts, asub-section 54 a and a sub-section 54 b, that include four spiralconductive traces 58.

The benefits provided by the present invention are substantial. Forexample, in a standard (straight) dipole antenna having an imaginarypart of 63 Ohms for an input impedance at 915 MHz, the length should bemore than 160 mm. However, using the embodiment in FIG. 2 (antenna 10),it is possible to achieve the same value with a length of only 50 mm anda width of only 8 mm, which is a considerable improvement over thestandard dipole.

In summary, the present invention relates to radio frequencyidentification (RFID) systems and more particularly to antennas for usein an RFID devices wherein the impedance more optimally matches with therelevant radio frequency (RF) circuitry in the transponder chip. Twomajor sections form the antenna, with one of them consisting of themainly radiating components, e.g. conductive metal traces, and the otherbeing substantially reactive components, e.g. consisting of spiral metaltraces. The sections are, directly or indirectly, connected, with themainly radiating components having suitable feed points for connectionto the transponder chip. Optionally, loop antenna characteristics can beadded by providing a loop section, in largely conventional manner, tothe folded dipole structure of the inventive antenna that has now beendescribed.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Thus, the breadth and scope of the invention should not belimited by any of the above described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. An antenna for use in a radio frequency identification (RFID)transponder, the antenna comprising: a first half portion and a secondhalf portion each including: a straight conductive section primarilydefining a radiating characteristic of the antenna; a spiral conductivesection creating a positive reactive characteristic of the antenna; saidstraight conductive section having a feed point where at the antenna canbe connected into the RFID transponder; and said straight conductivesection effectively connecting with its respective spiral conductivesection, thereby having said first half portion and said second halfportion characterize the antenna as being of a dipole type.
 2. Theantenna of claim 1, wherein said first half portion and said second halfportion are mirrored opposites that lie within a plane.
 3. The antennaof claim 1, wherein said first half portion and said second half portionare formed of printed circuit board traces.
 4. The antenna of claim 1,wherein said spiral conductive sections each include two opposedspiral-shaped sub-sections.
 5. The antenna of claim 1, furthercomprising a loop section connecting said straight conductive sectionsof said first half portion and said second half portion, thereby furthercharacterizing the antenna as being of a folded dipole type.
 6. Theantenna of claim 5, wherein said first half portion and said second halfportion are mirrored opposites and lie within a plane.
 7. The antenna ofclaim 5, wherein said first half portion and said second half portionare formed of printed circuit board traces.
 8. The antenna of claim 5,wherein said spiral conductive sections each include two opposedspiral-shaped sub-sections.
 9. An antenna for use in a radio frequencyidentification (RFID) transponder, the antenna comprising: first andsecond dipole means to radiated or receive radio frequency (RF) energy;said first and second dipole means each including straight conductivemeans to primarily define a radiating characteristic of the antenna;said first and second dipole means each further including spiralconductive means to create a positive reactive characteristic of theantenna; said straight conductive means each effectively connecting withits respective said spiral conductive means at one end and its other endincluding a feed point to connect the antenna to the RFID transponder.10. The antenna of claim 9, wherein said first and second dipole meansare mirrored opposites that lie within a plane.
 11. The antenna of claim9, wherein said first and second dipole means are formed of printedcircuit board traces.
 12. The antenna of claim 9, wherein said spiralconductive means each include two opposed spiral-shaped sub-sections.13. The antenna of claim 9, further comprising a loop means connectingsaid straight conductive means of said first dipole means and saidsecond dipole means, thereby further characterizing the antenna as beingof a folded dipole type.
 14. The antenna of claim 13, wherein said firstand second dipole means are mirrored opposites that lie within a plane.15. The antenna of claim 13, wherein said first and second dipole meansare formed of printed circuit board traces.
 16. The antenna of claim 13,wherein said spiral conductive means each include two opposedspiral-shaped sub-sections.